Semiconductor manufacturing of devices such as integrated circuits relies upon lithography to replicate a pattern onto components (e.g., wafers, substrates, microchips). A traditional optical lithographic system includes an exposure source, illumination optics, an object mask or reticle, photoresist and process methodology to transfer the pattern from a mask or reticle, to photoresist, and to the components. Although several types of lithography exist, optical lithography remains favored because of its precision and throughput in processing the components with minimum feature sizes. Current optical lithography utilizes short wavelengths (e.g., ultraviolet 193 nm) and high numerical apertures (e.g., Tropel Cheetah employs a numerical aperture (NA) of 0.9) to improve resolution during exposure.
Optical lithography nonetheless adds large cost to a finished semiconductor component, adding approximately one-third to the overall cost. As the operating wavelength decreases to 193 nm to produce reduced feature size, for example at 90 nm, the exposure source and optical components of the lithographic system also increase in complexity and cost. A move to entirely new infrastructures are required when the wavelength is changed.
The semiconductor industry relies heavily on optical lithography. Optical lithographic systems are used to manufacture integrated circuits (ICs) by replicating a pattern onto components (e.g., wafers, substrates, microchips). As the complexity of ICs increases, requirements to produce ever smaller features on the components (and hence in the patterns) are generated. The resolution of optical lithographic systems must therefore increase to allow continued production growth.
In prior art optical lithographic systems, global resolution improvements are made by using immersion with a very high numerical aperture (e.g., NA>1), and/or by decreasing the wavelength of the optical radiation employed. In immersion lithography, a liquid (e.g., ultra-pure water) is used between the lens and the component.
As NA increases, the depth of focus (DOF) decreases with the square of the increase in the NA. For example, from NA of 0.85 to 1.3 immersion, the DOF decreases by a factor of 2.3. As the wavelength of the optical radiation is reduced, the DOF decreases linearly with the decrease in wavelength. An increase in resolution is therefore not useful without adequate DOF.
Nonetheless, as DOF decreases, exposure tools will be required to control focus to an accuracy on the order of tens of nanometers. The reduced DOF raises other issues that include, for example, wafer flatness, wafer warping, thickness of the photo-resist, and reticle flatness. It is clear that the DOF must remain close to currently-used values, while resolution increases, if the performance of optical lithographic systems is to increase without dramatic rise in cost. For example, even though the tolerance on the reticle flatness is reduced by the square of any demagnification, the decrease in DOF requires flatter reticles, thereby increasing the cost of ‘mask blank’ (reticles prior to patterning) manufacture. Also, stress from chromium deposition during the patterning of the mask blank may also cause it to warp. A further consideration resulting from the decrease in DOF is gravitational effects on the reticle and wafer.
Problems may also occur during use of a reticle when the deposited chromium absorbs radiation, increasing its temperature and causing irregular thermal expansion. Periodic reticle realignment is frequently required, reducing productivity of the optical lithographic system. Non-correctable registration errors become significant for the 100 nm node and beyond.
Lenses form a major part of optical lithographic system cost. The aberrations of Petzval curvature and astigmatism increase proportional to the square of the NA; thus, higher quality (and more expensive) lenses are required to reduce these aberrations since the amount of permissible field curvature will be reduced due to the decrease in DOF. The lens' assembly tolerances are also reduced with a high NA. Thus DOF and focus related aberrations, e.g., field curvature, become major limitations of optical lithography. For example, the field of view of the imaging system, and hence the throughput of the lithography system, is necessarily reduced as NA increases.
With regard to image quality, the two-dimensional modulation transfer function (MTF) of a traditional lens is symmetrical and does not match the distribution of the spatial frequency information of a photo-mask or reticle being imaged. The spatial distribution of an integrated circuit reticle with Manhattan geometries has the bulk of the spatial frequency information along the horizontal and vertical spatial frequency axes. Accordingly, the transfer of the most important spatial frequencies must be maximized in the lithographic imaging system.
Since the 1980s, optical lithography has attempted to employ phase shift masks to improve this problems associated with DOF; however such efforts have not been successful due to increased complexity of the exposure source and optics within the lithographic system. To date, therefore, the prior art efforts to extend the depth of focus have been limited to 15% improvement.
Useful background to the following specification may be found in U.S. Pat. No. 5,348,837 and U.S. Pat. No. 5,748,371, each incorporated herein by reference.